That error helped, it pointed out the line number in the file with the issue. The problem was that Global Mapper wasn't properly escaping some characters (like ampersand) on export to OSM XML files. I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with this fixed for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

I have a similar problem: I opened an osm-file in GM V12, than, without any changes, did an export from GM to another osm-file. But this second file I cannot open in JOSM. Is there something wrong in my handling?
My GM-exe patch is from 11-13-2010, must be the newest one.

here are some additional hints for you:
In JOSM: The file *.osm could not be read. Error message: Row 4, column 63: Attribute 'version' for osm-element with ID 197909754 is missing (row 4, column 63).
In GM attributes seem to be ok and files can be reloaded.

Also forgot to mention there were some builds last week that were accidentally stripped attributes. There should be a new build with a build date from yesterday that should fix this. I have placed a new build at http://www.globalmapper.com/global_mapper12.zip with the change for you to try. Simply download that file and extract the contents into your existing v12.xx installation folder to give it a try. If you are using the 64-bit v12 version there is a new build at http://www.globalmapper.com/global_mapper12_64bit.zip .

unfortunately the fix was not successful. I loaded an osm-file from Potlatch as well as from the newest version of JOSM. When I load these osm-files in GM and export them in osm-format again (with or without changes) I cannot reload these files in JOSM. And I did identify, that GM makes the files much smaller, e.g. from 1,5 MB to 630 KB.

B U T: I am able to convert the osm-files (exported by GM) into a Garmin img-file with mkgmap, also then, if I did changes in GM! But nevertheless I would be glad, if I could open osm-files coming from GM in JOSM, too.

Global Mapper does not provide any special treatment to the "id" attribute that OSM data uses, so when you copy or create a new feature there isn't a unique "id" attribute provided. What you can do is either make sure to assign a unique "id" value when creating a new feature, or prior to exporting to OSM use the Search->Find Attributes with Duplicate Values menu command to find and automatically fix duplicate "id" attributes, then export to a new OSM file.